How to render height map in Blender?
I don't know the method you're using, here is one that seems to work and that is close to the one explained here by CGMatter as linked by @NascentSpace:

Create the 3D object you want to bake, put an Orthographic camera over it, give your object the following node setup: Texture Coordinate (Generated Output) &gt; Separate XYZ &gt; ColorRamp (if you need to tweak the highest and lowest) &gt; Emission &gt; Material Output:




In the Render panel, choose a good Sampling resolution. In the Output panel choose OpenEXR as File Format, otherwise you'll have artifacts. Keep the other settings as they are and render the image.




Create a plane, subdivide it, give it a new material with an Image Texture (in Non-Color mode) that you plug into the Height of a Displacement node that you plug into the Material Output Displacement socket. Load the OpenEXR image in the Image Texture node. In the Material panel &gt; Settings &gt; Surface &gt; Displacement, choose Displacement Only. It should work: